The number of moles of MgCl2 in 20.0 mL of a 0.800 M solution can be calculated using the formula:

Moles of MgCl2 = Concentration (M) x Volume of solution (L)

First, we need to convert the volume of the solution from milliliters (mL) to liters (L):

Volume = 20.0 mL = 20.0 / 1000 L = 0.0200 L

Now, we can substitute the concentration and volume into the formula:

Moles of MgCl2 = (0.800 mol/L) x (0.0200 L) = 0.0160 mol

Therefore, there are 0.0160 moles of MgCl2 present in 20.0 mL of a 0.800 M solution.